Auxiliary power units for providing auxiliary cooling, heating, and power to a vehicle are known. Auxiliary power units are often used by cross-country truckers. Trucks used by cross-country truckers are generally equipped with a sleeper compartment located behind a truck cab so that the trucker has a convenient place to sleep while enroute.
The truck has a main engine, hereinafter referred to as the truck engine, a heating system typically including a heater in the cab of the truck and a heater in the sleeper, and an air conditioning system typically including an evaporator for the cab of the truck and one for the sleeper. When the trucker parks the truck to rest or sleep, heating and air conditioning in the sleeper is provided by running the truck's engine. It is highly inefficient, however, to run the truck's engine merely to provide heating and air conditioning. It is much more efficient to use auxiliary power units, which usually have a small auxiliary engine, to provide power for heating and cooling.
In cold weather, trucker's often idle the truck's engine while the vehicle is parked. This is done to keep the truck's motor oil, diesel fuel, and engine warm so that it will restart. It is known to use an engine to keep the truck's engine warm by interconnecting the cooling system of the auxiliary engine to the truck's engine.
The auxiliary power units available can also generate electric power for various other truck accessories, such as lights and electric power receptacles.
Various approaches for providing auxiliary air conditioning, heating, and/or power are known. See, for example, U.S. Pat. No. 4,825,663 to Nijjar and U.S. Pat. No. RE 33,687 to Greer. In some of these approaches, a separate air conditioning system including its own engine, compressor, condensor, evaporator, and fan are installed. In yet other systems the truck's fan and evaporator are operated by an auxiliary engine having its own compressor and condenser.
In systems which use the truck's fan and evaporator, the truck's refrigerant lines and the auxiliary refrigerant lines are interconnected so that the auxiliary compressor can pump refrigerant through the truck's refrigerant lines. This means that both the auxiliary refrigerant compressor and the truck's refrigerant compressor are on the same system of refrigerant lines. With this arrangement, one of the refrigerant compressors would be seriously damaged if both compressors are run simultaneously. To prevent damage to one of the compressors, a switching mechanism has been used in auxiliary power units, prior to the present invention, so that when one compressor is activated, the other is turned off. Even with the switching mechanism, however, having two compressors on the same system of refrigerant lines can lead to failure of one of the compressors because lubricant in the refrigerant tends to migrate and collect in one of the compressors, thereby under-lubricating the other. Moreover, if a leak develops in the refrigerant lines, this might result in a loss of refrigerant for the entire system. Additionally, if the evaporator of the truck's air conditioning system is inoperable, air conditioning cannot be provided by operation of the truck's engine or the auxiliary engine.
When installing auxiliary power units, prior to this invention, a plurality of auxiliary coolant lines were interconnected with a plurality of standard coolant lines in the truck. The interconnection was provided so that the auxiliary power unit could provide heated coolant to the heating units within the truck by way of the truck's coolant lines.